Combustors are commonly used in industrial and power generation operations to ignite fuel to produce combustion gases having a high temperature and pressure. For example, gas turbines typically include one or more combustors to generate power or thrust. A typical gas turbine used to generate electrical power includes an axial compressor at the front, one or more combustors around the middle, and a turbine at the rear. Ambient air may be supplied to the compressor, and rotating blades and stationary vanes in the compressor progressively impart kinetic energy to the working fluid (air) to produce a compressed working fluid at a highly energized state. The compressed working fluid exits the compressor and flows into a combustion chamber where the compressed working fluid mixes with fuel and ignites to generate combustion gases having a high temperature and pressure. The combustion gases expand in the turbine to produce work. For example, expansion of the combustion gases in the turbine may rotate a shaft connected to a generator to produce electricity.
Various design and operating parameters influence the design and operation of combustors. For example, higher combustion gas temperatures generally improve the thermodynamic efficiency of the combustor. However, higher combustion gas temperatures also promote flashback or flame holding conditions in which the combustion flame migrates towards the fuel being supplied by fuel nozzles, possibly causing severe damage to the fuel nozzles in a relatively short amount of time. In addition, higher combustion gas temperatures generally increase the disassociation rate of diatomic nitrogen, increasing the production of nitrogen oxides (NOX). Conversely, a lower combustion gas temperature associated with reduced fuel flow and/or part load operation (turndown) generally reduces the chemical reaction rates of the combustion gases, increasing the production of carbon monoxide and unburned hydrocarbons.
In a particular combustor design, one or more late lean injectors or tubes may be circumferentially arranged around the combustion chamber downstream from the fuel nozzles. A portion of the compressed working fluid exiting the compressor may flow through the tubes to mix with fuel to produce a lean fuel-air mixture. The lean fuel-air mixture may then be injected by the tubes into the combustion chamber, resulting in additional combustion that raises the combustion gas temperature and increases the thermodynamic efficiency of the combustor.
The late lean injectors are effective at increasing combustion gas temperatures without producing a corresponding increase in the production of NOX. However, the tubes that provide the late injection of the lean fuel-air mixture typically have a substantially constant cross section that creates conditions around the late lean injectors susceptible to localized flame holding. In addition, the tubes are generally aligned perpendicular to the flow of combustion gases in the combustion chamber. As a result, the late lean injectors may produce large vortices that recirculate hot combustion gases back to the surface of the combustion chamber, producing high thermal gradients and shortening hardware life. Therefore, an improved system for supplying working fluid to the combustor that reduces the conditions for flame holding and/or vortex shedding would be useful.